The gasification of coal and other carbon-based resources is a versatile conversion technology. During gasification, feedstocks are transformed into a synthesis gas (syngas) generally comprising a mixture of H2, CO and CO2 which may be utilized in a variety of downstream processes. For example, the syngas may be used as a fuel in integrated gasification combined cycles (IGCC), or as a feedstock for producing H2 or other hydrocarbon fuels. Syngas can also be used as a feedstock for a number of chemical processes, including Fischer-Tropsch synthesis, methanation, and methanol and ammonia production. However, conventional coal gasification processes are generally capital intensive and require significant amounts of parasitic energy. Typically they involve partial coal combustion with either O2 or air in concert with a catalyst to promote gasification reactions. When air is utilized, N2 can enter the syngas, diluting the syngas and extraction is difficult. When O2 is utilized, expensive oxygen production units tend to generate high parasitic losses. As a result, the development of alternative methods for clean coal gasification are a significant area of current interest.
Chemical-looping gasification (CLG) of coal is one alternative method. CLG utilizes oxygen carriers to transfer oxygen and react with coal, which is partially oxidized into synthetic gas consisting of CO and H2. Oxygen carriers avoid direct contact with air and act as catalysts for gasification reactions. Synthesis gas produced by coal gasification with steam and oxygen carriers can be used for many important applications since the gas stream is free of N2, as indicated at reaction [1].C+Oxygen carrier+steam→H2+CO+CO2(No N2)  [1]
This N2-free synthesis gas can be used to produce pure H2 as shown in reaction scheme [2].H2+CO+CO2→Water gas shift reaction (WGS)→H2+CO2→Traditional pressure swing adsorption to separate CO2→Pure H2  [2]
A synthesis gas stream without N2 with catalysts can also be used to produce useful chemicals as shown in reaction scheme 3 and 4.H2+CO+CO2→methanol→plastics, adhesives and fuels  [3]H2+CO+CO2→fuels via Fischer-Tropsch synthesis or dimethyl ether  [4]
However, gasification of solid fuels such as coal with oxygen carriers is a challenging process. In a fluidized-bed reactor system in which solid-solid interactions are minimal, all the reactions mainly proceed via gaseous species. For instance, steam is utilized for initial gasification of coal to produce synthesis gas, and when an oxygen carrier is present the synthesis gas is further oxidized to form CO2 and H2O to complete combustion as shown in reactions [5] and [6].Coal+steam→CO+H2  [5]CO+H2+metal oxide→CO2+H2O+reduced metal oxide  [6]
Therefore, when the oxygen carrier is present it is usually difficult to control the reaction at the steam gasification stage (reaction 5), preventing the combustion reaction 6.
To use oxygen carriers for direct gasification of coal, it would be useful to identify an oxide which was not reduced by reaction [6], but could still react with the solid coal, or coal volatiles. In this scenario, reaction of coal with the oxygen carrier has to occur via solid-solid reaction to form synthesis gas.
It would be advantageous to develop an oxygen carrier for gasification which reacting directly with coal to form synthesis gas which having a minimal reactivity with the synthesis gas. One of the more important criteria for CLG systems is that synthesis gas produced by the reaction with coal/oxygen carrier should not further react with the oxygen carrier, because it is necessary to avoid combustion of fuel and stop the reaction at the gasification stage.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.